The Stirling-cycle engine was first conceived and reduced to practice in Scotland 164 years ago. A hot-air, closed-cycle prime mover based on the principle was patented by the Reverend Robert Stirling in 1817 as an alternative to the explosively dangerous steam engine. Incredibly, this event occured early in the Age of Steam, long before the invention of the internal combustion engine and several years before the first formal exposition of the Laws of Thermodynamics.
Air was the first and only working fluid in early 19th century machines, whereas hydrogen and helium have been the preferred working fluids for modern machines. In Great Britain, Europe, and the United States thousands of regenerative hot air prime movers in a variety of shapes and sizes were widely used throughout the 19th century. The smaller engines were reliable, reasonably efficient for their time, and most important, safe compared with contemporary reciprocating steam engines. The larger engines were less reliable, however, because they tended to overheat and often succumbed unexpectedly to premature material failure.
Toward the end of the 19th century the electric motor and the internal combustion engine were developed and began to replace not only the Stirling-cycle engines, but also the reciprocating steam engines of that era. These new machines were preferred because they could produce greater power from more compact devices and because they were more amenable to simple power level controls. Both of these limitations of the original Stirling engine are directly attributable to the use of air as the working fluid. That is, the specific power capacity and the overall mechanical complexity of a practical Stirling-cycle machine are direct consequences of the inherent performance characteristics and heat transfer properties of the working fluids used.
Since World War II there have been unprecedented adances in the general technologies of machine design, heat transfer, materials science, system analysis and simulation, manufacturing methods, and Stirling engine development. Today, in comparison to their conventional internal combustion counterparts, all modern Stirling-cycle prime movers are external combustion engines which consistently demonstrate (in the laboratory) higher efficiency, multifuel capability, lower exhaust emissions, quieter operation, equivalent power density, and superior torque characteristics. Nevertheless, none of these engines is mass produced for any commercial application anywhere in the modern world.
The reason for this is that contemporary Stirling engines have been developed largely by adapting traditional methods and designs from the more familiar internal combustion engine technology base. Patchwork adaptation of the old as a shortcut path to the new is a process which inexorably produces a hodge-podge arrangement of excessive mechanical complexity and which inevitably results in high production costs.
The use of hydrogen and helium has led to increased power density, but it has also required the use of complex seals, makeup storage vessels, expensive materials, and elaborate controls. Hydrogen, in particular, is difficult to contain and causes ordinary engine materials to become brittle. Despite clearly superior technical performance characteristics, therefore, contemporary Stirling engines are invariably not cost competitive from the standpoint of economical mass production.
The invention comprises fundamental concepts and material properties which are used in combination to form a new and less complicated technology base for the development of improved Stirling-Cycle machines, specifically including the following: (1) working fluids other than hydrogen, helium, or air, namely certain fluorine compounds exemplified by sulfur hexafluoride, perfluorobutane, perfluoropropane, and octafluorocyclobutane, which provide an increased dynamic heat transfer coefficient yet are nonflammable, non-toxic, and easily liquefied; and (2) an engine power level control subsystem by which the mean system working pressure, and thereby the instantaneous power level of the engine, is conveniently varied by the hydraulic injection or ejection of condensed working fluid through a special heat exchanger to be known as the reservoir cooler.
It is a primary object of the invention to provide a new and improved family of thermodynamic working fluids for Stirling-cycle, reciprocating, thermal machines, other than the usual hydrogen, helium, or air which possess increased dynamic heat transfer coefficients; have a critical temperature somewhat above the minimum ambient temperature of the available heat sink yet somewhat below the designated heat rejection temperature of the cycle as maintained within the engine cooler; and which are also nonflammable, nontoxic, and inexpensive, inert, and of low viscosity.
It is a further object of the invention to provide an engine power level control subsystem for use in conjunction with any regenerative thermal machine by means of which the nominal operating pressure of the cycle is conveniently and automatically varied as a direct function of power demand by the hydraulic injection or ejection of condensed working fluid through a special heat exchanger to be known as the reservoir cooler.